Preheat your waffle iron. 

In a large bowl, stir together flour, baking powder, salt, and sugar.

In a separate bowl, whisk together milk, egg yolks, and vanilla. Pour wet ingredients over flour mixture, and gently stir together. 

Slowly pour in the melted butter, reserving about a tablespoon, and continue mixing until gently combined.

Using a stand mixer or hand-held mixer, beat the egg whites until stiff peaks form. This will take a few minutes.

Slowly fold the the egg whites into the waffle batter.

Dip a pastry brush into the reserved melted butter and coat the waffle iron lightly. 

Add in batter according to your waffle iron (for mine, we use a scant 1/2 cup of batter.)

Cook as directed by your waffle iron. 

Remove and serve immediately. They are delicious plain, or try topped with powdered sugar, whipped cream and berries, or a bit of jam!